diff --git a/mercurial/debugcommands.py b/mercurial/debugcommands.py
--- a/mercurial/debugcommands.py
+++ b/mercurial/debugcommands.py
@@ -3865,11 +3865,14 @@
 def debugtagscache(ui, repo):
     """display the contents of .hg/cache/hgtagsfnodes1"""
     cache = tagsmod.hgtagsfnodescache(repo.unfiltered())
+    flog = repo.file(b'.hgtags')
     for r in repo:
         node = repo[r].node()
         tagsnode = cache.getfnode(node, computemissing=False)
         if tagsnode:
             tagsnodedisplay = hex(tagsnode)
+            if not flog.hasnode(tagsnode):
+                tagsnodedisplay += b' (unknown node)'
         elif tagsnode is None:
             tagsnodedisplay = b'missing'
         else:
diff --git a/tests/test-tags.t b/tests/test-tags.t
--- a/tests/test-tags.t
+++ b/tests/test-tags.t
@@ -444,12 +444,12 @@
 
   $ hg debugtagscache | tail -2
   4 0c192d7d5e6b78a714de54a2e9627952a877e25a 0c04f2a8af31de17fab7422878ee5a2dadbc943d
-  5 8dbfe60eff306a54259cfe007db9e330e7ecf866 0c04f2a8deadde17fab7422878ee5a2dadbc943d
+  5 8dbfe60eff306a54259cfe007db9e330e7ecf866 0c04f2a8deadde17fab7422878ee5a2dadbc943d (unknown node)
 
   $ rm -f .hg/cache/tags2-visible
   $ hg debugtagscache | tail -2
   4 0c192d7d5e6b78a714de54a2e9627952a877e25a 0c04f2a8af31de17fab7422878ee5a2dadbc943d
-  5 8dbfe60eff306a54259cfe007db9e330e7ecf866 0c04f2a8deadde17fab7422878ee5a2dadbc943d
+  5 8dbfe60eff306a54259cfe007db9e330e7ecf866 0c04f2a8deadde17fab7422878ee5a2dadbc943d (unknown node)
 
   $ hg tags
   abort: data/.hgtags.i@0c04f2a8deadde17fab7422878ee5a2dadbc943d: no match found